Research during the past year focused on defining the molecular mechanisms underlying the development of the mammalian olfactory system. Particular emphasis was placed on the identification of molecules which direct the establishment of the precise patterns of synaptic connectivity observed between peripheral sensory neurons and their target neurons in the olfactory bulb. These studies led to the identification of novel members of the semaphorin gene family that are highly expressed in the olfactory and vomeronasal systems. Full length cDNA clones for several of these genes were isolated and sequenced. In situ hybridization experiments indicate that members of this gene family are expressed early in development during the time when synaptogenesis between primary sensory neurons and bulbar mitral cells is occurring. The genes encoding neuropilin 1 and 2, the two identified semaphorin receptors, were also found to be expressed in the olfactory and vomeronasal systems early in development. Of particular interest, we found that particular semaphorins and their receptors were differentially expressed in the accessory and main olfactory systems or in different zones of the olfactory epithelium. The functional significance of the differential distribution of these molecules is being investigated. A second focus of research was the molecular characterization of odorant receptors. In order to examine the functional properties of odorant receptors including ligand specificity, G- protein coupling and selectivity, and ligand-dependent modifications, we are currently developing a heterologous cell expression system. Mammalian vectors containing the complete coding regions of several odorant receptors were constructed. Stable and transiently transfected cell lines producing high levels of odorant receptor mRNA are currently being evaluated for the production of functional odorant receptors.